Tips on using wire wound Inductor as ST25DV antenna.

[RoastedMosfets]

Hello,

Looking to integrate the ST25 in an ultra space constrained environment. As such antenna space is very limited so it seems like the best solution would be a wire wound inductor as read distance is not supercritical.

The Pulse W3102 4.7uH inductor seems to be designed to be a NFC antenna. What are somethings I should keep in mind when integrating this as the ST25DV antenna? Will any additional tuning components be need? Would any wirewound 4.7uH inductor work? 4.7uH seems to be a fairly common value for NFC wirewound antennas, why is that?

Additionally due to some of the safety requirements for this device there may be a design constraint to keep this inductor at 1uH or below. Preliminary research seems to indicate that a 1uH NFC should still be fine (?) . Could integrating a 1uH inductor work? What tuning components will be needed (at least to try as a first pass) ?

Thanks.

[RoastedMosfets]

bump, if anyone from ST has any thoughts. Thanks.

[Henry Crane]

Dear,

Sorry for our late answer.

first, here is attached a fomer application note dealing with the use of smd inductors as antennas for our ISO 15693 M24LR product. It is also applicable to M24LRE-R and ST25DV product families.

Tuning capacitance for M24LR(E) and ST25DV product families is typically 28.5pF. To reach a tuning frequency between 13.56 and 14MHz as we recommand for ISO 15693 or NFC-V products, the coil antenna inductance has to be close to 4.7µH.

This comes from the formula fr=1/(2.pi.sqrt(L.C)) where C is 28.5pF and L=4.7µH and fr=13.75µH is the resonant frequency of the LC circuit.

If you want use a 1µH smd coil maintaining the same resonant frequency, you'll need to balance the coil inductance by adding external caps in parallel to the chip. Dividing the inductnace by 4.7 requires the new total chip capacitance (28.5pF + external capacitance) to be 28.5pF x 4.7. As a result, the total additional extarnal capacitance value will be (28.5pFx4.7)-28.5pF=3.7x28.5pF= 105.5pF.

Care must be taken that adding external ceramic smd capacitors with such large value compared to chip tuning cap results in a significant increase of the Q factor of the chip. The consequence of this is an increase of rise time and fall time of the reader modulation reaceived on the AC0-AC1 input of the chip. When using the 100% reader to tag modulation index, please check that amplitude modulation remains at "0%" at least for 2.1µS during reception of reader command on AC0-AC1 signal.

this can be done probing the AC0-AC1 signal with a differential oscilloscope probe.

Chosing a smd coil inductance downto 3.8µH (and use relevant external tuning cap), as a rule of thumb, does not require such check.

best regards,

Henry Crane, RFID/NFC product support team.

[RoastedMosfets]

Hi Henry,

Appreciate the reply. This is really insightful!

Is a ground plane cutout under the inductor necessary?

Because we are ultra space constrained could this still work without a ground plane cutout.

Any idea on how much of a minimum additional keepout area would be needed?

Or just a keepout under the device would be ok. I understand typically with more typical antennas we'd want at least an additional couple of millimeters at a minimum. 

Right now the inductor is going to be mounted near the center of the PCB with ground plane all around it. 

Thanks for your help.

[Henry Crane · Accepted solution]

​Hi,

we generally recommand to place the SMD coils in a corner of the PCB, located in an area were there no copper, typically a square area with a side equal to the diameter of the coil plus 1 or 2 mm. the reason of this placement is that the PCB is a shield for the magnetic field flowing into the smd coil but also the reader. If you place the coil in the middle of a PCB which dimension is in a range or larger than the reader antenna, eddy currents flowing on the surface of the PCB will cancel the magnetic field coming from the reader with the result that the field flowing to ST25DV coil  can be lowered compared to free space situation. Placing the smd coil in a corner of the PCB  causes the pcb copper plane to block the reader field over a reduced surface and ease tag powering and communications.

this said, you can estimate the impact by placing the smd coil you plan to use, soldered to a ST25DV sample, in the middle of a copper sheet corresponding to the side of your PCB opening the copper area below smd coil with a diameter equal to the smd coil diameter plus 1,2,mm or more. the more you leave space around the coil the better it is.

best regards,

Henry.